The present disclosure relates to a light-emitting device, and more particularly, to a light-emitting device for a headlight.
Semiconductor light-emitting elements, such as light-emitting diodes and laser diodes, have been brought into use as various light sources, and in recent years particularly, as light sources for vehicle headlights. These light sources for vehicle headlights are required to show, for example, a specific light distribution called a Lambertian light distribution, in addition to high luminance. As a light-emitting device suitable for a headlight, Japanese Patent Publication No. 2015-76455 discloses a light-emitting device that includes semiconductor light-emitting elements disposed on or above the surface of a substrate, a light-reflective frame enclosing the semiconductor light-emitting elements on or above the substrate, and a transparent phosphor layer covering the upper and lateral surfaces of the semiconductor light-emitting elements. This light-emitting device emits light in one direction from the opening of the frame because light is reflected off the inner circumferential wall surfaces of the light-reflective frame. The light-emitting device thus provides an enhanced illuminance on the front of an automobile and is deemed to be suitable for a headlight.
Also, it is required to prevent a vehicle headlight in low beam mode from dazzling a driver of an oncoming vehicle by cutting off light at the upper side while ensuring illumination of the road surface. The boundary of the light at the upper side that is cut off in low beam mode is called a cut-off line 210. It is preferable for safety reasons that the cut-off line 210 be clear, that is, that the upper side of the cut-off line 210 not be irradiated with light in low beam mode (FIG. 14). Accordingly, the optical system of a vehicle headlight is designed such that the cut-off line 210 is clear in low beam mode, and a light-emitting device used as a light source for a headlight is also required to have such luminance characteristics that allow for an optical system offering a clear cut-off line 210 can be designed. Specifically, the light-emitting device is required to have, for example, a clear boundary between a light-emitting surface and a light-reflective frame (i.e., covering member) surrounding the periphery of the light-emitting surface, that is, to have such characteristics that the luminance differs sharply between the outer and inner sides of the boundary as shown in FIG. 13. To make the boundary between the light-emitting surface and the covering member surrounding the periphery of the light-emitting surface clear, as schematically shown in FIG. 15, it is effective to provide a large wavelength conversion member 207 to be disposed on light-emitting elements 201, dispose the wavelength conversion member 207 on the light-emitting elements 201 such that the periphery of the wavelength conversion member 207 is located outside the peripheries of the light-emitting surfaces of the light-emitting elements 201, and dispose a covering member on the light-emitting elements 201 such that the wavelength conversion member 207 is surrounded.
However, the light-emitting surface of the light-emitting device inevitably becomes large if a phosphor plate (i.e., wavelength conversion member) is disposed on or above the light-emitting elements such that the periphery of the phosphor plate is located outside the peripheries of the light-emitting surfaces of the light-emitting elements. Thus, there is a problem that the expanded light-emitting surface of the light-emitting device makes the structure of the optical system of a headlight large.
Accordingly, an object of certain embodiments described herein is to provide a light-emitting device that can offer a clear cut-off line using a simple and small optical system when the light-emitting device is used as a light source for a headlight.